Memory devices may be provided in apparatuses such as computers or other electronic devices, including but not limited to portable memory devices, solid state drives, personal digital assistants, music players, cameras, phones, wireless devices, displays, chip sets, set top boxes, gaming systems, vehicles, and appliances. There are many different types of memory including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), flash memory, and resistance variable memory, among others.
Apparatuses, such as resistance variable memory devices, may be used as non-volatile memory for a wide range of electronic devices. Resistance variable memory devices may include, for example, phase change random access memory (PCRAM) or resistive random access memory (RRAM), among others. A physical layout of a PCRAM device may resemble that of a DRAM device where the capacitor of the DRAM cell is replaced by a phase change material, e.g., Germanium-Antimony-Telluride (GST) or other chalcogenide materials. That is, an access device such as a diode or metal oxide semiconductor field effect transistor (MOSFET) can be connected in series with the phase change material. Chalcogenide materials can include compounds of sulfides, selenides, or tellurides, among others. GST has been used in rewriteable optical discs, e.g., rewritable compact discs (CD-RW) and rewritable digital versatile discs (DVD-RW).
A physical layout of an RRAM device may include memory cells including a dielectric exhibiting sufficient switching properties. The thin film can be connected to an access device such as a diode, a field effect transistor (FET), or a bipolar junction transistor (BJT). Generally, an RRAM device may include resistance variable memory elements that may include any dielectric material exhibiting sufficient switching properties. Example dielectrics include perovskites, transition metal oxides, chalcogenides, and silicon dioxide.
Memory cells, including resistance variable memory cells, can be programmed to one of a plurality of resistance states. The resistance of a PCRAM memory cell can be altered by applying energy pulses to the phase change material, e.g., GST. For example, material properties of the GST may be altered by heating it with a programming current. Generally, a higher resistance state may be associated with a more amorphous state of the phase change material, and a lower resistance state may be associated with a more crystalline state of the phase change material.
The resistance of an RRAM memory cell can be increased and/or decreased by applying positive and/or negative electrical pulses across the film. Generally, a voltage pulse of a sufficiently high magnitude provided (e.g. applied) to a resistance variable memory element may cause a conduction path, e.g. one or more filaments, to form through an otherwise dielectric, e.g. insulating, material. Once formed the filament may be reset (e.g. broken, yielding a high resistance) or again set (e.g. reformed, yielding a low resistance). High resistance and low resistance states, as used herein, refer to states of a memory cell having a detectable difference in resistance.
To sense data stored on a resistance variable memory cell, the resistance of the memory cell may be sensed. In a binary system, a low resistance may correspond to a first data value, e.g., 0, and a high resistance may correspond to a second value, e.g., 1. In some binary systems, a low resistance may correspond to a data value of 1 while a high resistance corresponds to a data value of 0. During a sensing operation, a sensed voltage and/or current corresponding to a resistance of a selected memory cell may be compared to a reference voltage and/or current to determine the content of stored data. That is, the resistance of the selected memory cell may be sensed indirectly. For example, a transient response of a sensing circuit that is connected to the selected memory cell may be sensed, e.g., in response to a change in voltage or current.
A single level cell (SLC) can be programmed to one of two resistance states, each corresponding to one of the binary digits 1 or 0. Memory cells can also store more than one digit of data, e.g., 1111, 0111, 0011, 1011, 1001, 0001, 0101, 1101, 1100, 0100, 0000, 1000, 1010, 0010, 0110, and 1110. Such cells may be referred to as multi state memory cells, multidigit cells, or multilevel cells (MLCs). MLCs can allow the manufacture of higher density memories without increasing the number of memory cells since each cell can be programmed to one of more than two resistance states, e.g., each corresponding to more than one bit of data. Some non-volatile memories, such as flash, may achieve MLC functionality by storing one of a range of charges on a floating gate memory cell. Resistance variable memories may achieve MLC functionality by programming a memory cell to one of multiple detectable resistance states.